Huge Asteroid Wrapped in Thick Dust Blanket

If
astronauts ever visit the asteroid Lutetia, they may have to strap on
snowshoes to avoid sinking into its nearly half-mile-thick layer of
dust.

Dusty
debris shrouds the huge asteroid to a depth of at
least 2,000 feet (600 meters), scientists have calculated. The
dust probably resembles the regolith found on the
moon, and it's a result of the intense cosmic
pummeling Lutetia has endured from other space rocks since the birth
of the solar system. [Photo
of Lutetia craters.]

"It
must have been produced by impacts," said Rita Schulz of the
European Space Agency in a media briefing yesterday (Oct. 4) in
Pasadena, Calif. The announcement came at a conference organized by
the American Astronomical Society's Division of Planetary Sciences.

A
visit to Lutetia

The new
look at Lutetia,
which is located in the asteroid belt between Mars and Jupiter, is
the result of a close fly-by made by ESA's
Rosetta spacecraft in July. Rosetta, whose chief
task is chasing
down and studying a comet,
zoomed to within 1,900 miles (3,162 km) of Lutetia, making it the
largest asteroid ever to be visited by a space probe.

Rosetta
snapped some detailed photos and probed the space rock from afar with
a suite of instruments. The data confirmed that Lutetia is an
elongated body with its longest side spanning approximately 81 miles
(130 km), ESA officials have said.

But it
took some time for scientists to process and analyze much of
Rosetta's data. Over the last few months, they scrutinized some of
Lutetia's many craters, measuring how deep they are. Researchers then
compared that information with predictions of how deep such craters
are expected to be based on theoretical models.

The
craters were much shallower than researchers had anticipated, Schulz
said, because of the thick layer
of dusty debris. Indeed, one of Rosetta's most dramatic photos shows
a landslide
of dust at the bottom of an enormous crater.

Based
on Lutetia's size, Schulz said, the asteroid
probably has a relatively high escape velocity 
the speed needed to overcome the
rock's gravitational pull. So about 90 percent of
the debris ejected by Lutetia's many collisions with other cosmic
bodies probably falls back onto
the space rock's surface. Over the course of 4.6
billion years or so, this would add
up to a pretty hefty dust coating.

Scientists
think the regolith covering the moon was produced the same way.

A
solid piece of rock

Rosetta
has revealed other intriguing details about Lutetia. Using data
gathered by the probe's instruments, scientists were able to estimate
the asteroid's density. And the results suggest Lutetia is a solid
piece of rock, not a loosely agglomerated "rubble
pile" that many
other asteroids are suspected to be.

Rosetta
also studied Lutetia's color.

"It
is boringly gray," Schulz said. But that doesn't mean the
asteroid is any less interesting, she added. "That means it's
like the moon, which isn't boring at all."

Schulz
said more Lutetia revelations will be forthcoming after the research
team spends more time with Rosetta's data.